An exhaust system conducts hot exhaust gases generated by an engine through various exhaust components to reduce emissions and control noise. The exhaust system includes an injection system that injects a diesel exhaust fluid (DEF), or a reducing agent such as a solution of urea and water for example, upstream of a selective catalytic reduction (SCR) catalyst. A mixer is positioned upstream of the SCR catalyst and mixes engine exhaust gases and products of urea transformation. The injection system includes a doser that sprays the urea into the exhaust stream. The urea should be transformed as much as possible into ammonia (NH3) before reaching the SCR catalyst. Thus, the droplet spray size plays an important role in reaching this goal.
The industry is moving towards providing more compact exhaust systems, which results in reduced volume of the system. Systems that spray larger size droplets may not be able to provide adequate transformation of urea when used in more compact system configurations. As such, smaller droplet size dosers are required for these more compact configurations to provide a more effective transformation into ammonia. However, the spray generated by small droplet dosers is very sensitive to recirculation flow, which can lead to increased deposit formation.
Further, as known, back pressure within the mixer increases as the volumetric exhaust flow through the mixer increases. Increased back pressure provides the benefit of reduced deposit formation. The risk for deposit formation is higher at engine operating points that have low volumetric flow and thus lower back pressure generated by the mixing system. The deposits can build up over time and can adversely affect system operation.